Details of the CATV modulator are well known to persons skilled in this art and can be located from many prior documents describing the operation of these arrangements. Extensive details are available in the Technical Reports available from Cable Television Laboratories Inc and particularly the reports entitled
Data-Over Cable Service Interface Specifications Modular Headend Architecture which is CM-TR-MHA-V02-081209 copyright 2008; and
Data-Over Cable Service Interface Specifications which is CM-TR-CMAP-V01-101222 copyright 2010.
These documents are published and available on line from their web site at Cablelabs.com. The disclosure of the above documents is hereby incorporated by reference.
Traditional approaches to full-band CATV/QAM coverage involve:
a) a single or dual-stage (heterodyne) upconversion architecture, modulating one or more channels from IF (Intermediate Frequency) to RF (Radio Frequency) with fully agile local oscillators (LO) covering the desired output band, or
b) a “direct RF” method making use of a digital to analogue converter (DAC) whose sample rate is greater than 2× the desired output bandwidth to absorb the upconversion function into the digital domain, or
c) a direct quadrature (IQ) upconversion architecture, making use of a complex modulation to upconvert one or more channels from baseband to RF.
Common challenges or drawbacks to the single or dual-stage upconversion involve isolation of the LO stages, noise, spurious, and return loss performance, size and complexity of circuitry, power consumption and cost.
By absorbing the upconversion into the digital domain, the direct RF approach reduces component complexity and isolation concerns but introduces distortions and noise that fold back from the DAC clock rate by an amount proportional to the highest channel frequency of the system. The DAC clock rate must be pushed out to 2× or 4× the desired output bandwidth to move these components outside of the output band. Additionally, the broadband noise performance of the system is dictated by the noise floor of the DAC itself. Since the DAC output is full-band, no further noise filtering is possible.
Direct IQ improves on power and bandwidth efficiency by making use of both the positive and negative-frequency spectrum through zero-IF image rejection upconversion. In-phase and quadrature-phase signals are separately upconverted and combined at RF to enable the use of the negative-frequency spectrum assembled by the modulator at baseband. As a result, for a given DAC clock frequency and system power, direct IQ achieves a two-to-one increase in efficiency.